The hypothesis that South African game animals that use browse containing condensed tannins (CT) could have rumen microbial populations better able to ferment tanniniferous forages was explored. Rumen fluid (RF) from a range of browsing ruminants was used to ferment tanniniferous forages in vitro and the results were compared with the values obtained with rumen fluid from sheep. Leaves of the shrub legumes Leucaena leucocephala, L. pallida, L. trichandra, Calliandra calothyrsus, Gliricidia sepium, and Acacia boliviana were used as substrates. In vitro digestibility of dry matter (IVDMD) and nitrogen (IVND) was measured in the presence or absence of polyethylene glycol (PEG) 4000 to assess the adverse effects of the CT in the feeds. Rumen fluid from giraffe, greater kudu, eland, duiker, impala, nyala, goat, and gnu was compared with that from sheep. Ability to metabolise or block the adverse effects of CT was expected to result in only a small or no response to PEG in terms of IVDMD and IVND with rumen fluid from browsers, and therefore to result in a significant animal × feed and animal × PEG interaction.
There were no significant (P > 0.05) animal × feed interactions. For every animal species there was a response to PEG in terms of IVND, and this response was linearly related to the CT in the feed as measured by PEG binding using 14C-labelled PEG. Rumen fluid from browsers had a lower pH (5.78 ± 0.060 v. 6.68 ± 0.052) and a higher dry matter content (1473 ± 102 mg v. 415 ± 4.3 mg/100 mL RF) than that of grazers (sheep and gnu). However, when measured at 2 pH levels (6.7 and 5.8), IVDMD and IVND of the feeds were not improved at the lower pH with rumen fluid from the browsers. Rather, digestibility was depressed at the lower pH with rumen fluid from all animal species.
We conclude from these in vitro studies that rumen microbial populations from browsers are not able to ferment tanniniferous forages better than rumen microbial populations from grazers. It seems likely that these browsers do not have rumen bacteria capable of degrading the CT in these shrub legumes. Production on such tanniniferous feeds may well depend on tannin binding with proline-rich saliva rather than on metabolism of or tolerance to CT by rumen bacteria.